1. Field of the Invention
The present invention relates to electronic devices and more specifically to electronic devices containing a single-sided circuit board.
2. Description of the Related Art
In designs for electronic devices for motor vehicles, functionality is of course very important. However, electromagnetic compatibility with the various electronic systems on the vehicle is also of great importance. One aspect of electromagnetic compatibility is assuring that all devices on the vehicle are within acceptable limits for radiated emissions.
Electronic devices having printed circuit boards conventionally have "double sided" circuit boards. That is, the circuit board of such devices has circuit traces on both sides. This provides a significant electromagnetic-compatibility benefit in that the routing of signals among components on the circuit board can be kept as short as possible. This is true because a circuit trace on the board can run partially on one side of the board, and then be routed through a hole and continue its run on the other side of the board, to provide substantial flexibility to minimize the length of the run. Also, having a double-sided board allows for generous provision of ground traces on both sides of the board, to allow ready access to a ground trace for return of signal current. In allowing signal routing and return to be kept as physically short and routed as closely together as possible, a double-sided circuit board minimizes the opportunity for the signals to radiate to the environment away from the circuit board.
As attractive as a double-sided circuit board may be, however, it has a very substantial disadvantage: cost. In an electronic device such as an instrument cluster for a motor vehicle, a double-sided circuit board may be the most expensive component, in the instrument cluster (even when taking into account a microprocessor, which is becoming a frequent inclusion in instrument clusters).
One particular type of instrument cluster coming into more frequent use is a cluster which has stepper motor gauges. Such gauges, as their name implies, use a stepper motor to position the gauge pointers of the instrument cluster. Such clusters almost invariably include a microcontroller. Microcontrollers having integrated capability to drive a plurality of stepper motor gauges are available.
A microprocessor, which typically operates at clock frequencies of several megahertz, poses a significant radiated emissions challenge. In a microcontroller having integrated stepper motor gauge drive capability, regulated supply voltage is supplied to several terminals and used for several purposes. For example, a regulated five-volt supply input (and a corresponding regulated five-volt return) is provided to power the "core" of the microcontroller. The "core" is the typical digital microprocessor section of the microcontroller. Regulated five-volt supply inputs (and a corresponding regulated five-volt return for each) are typically also provided to power the stepper motor control outputs of the microcontroller, general purpose inputs and outputs of the microcontroller and analog power for the microcontroller.
The current drawn and returned by the microcontroller to and from the voltage regulator on the circuit board can provide a significant radiated emissions challenge. A known design to minimize radiated emissions includes using a two-sided circuit board. A five-volt regulator on the circuit board is coupled to a single node from which several five-volt supply traces are "starred" to the various microcontroller inputs requiring five-volt regulated power. This single node is coupled by a capacitor to ground. In the star configuration, the various supply traces to the microcontroller inputs, being run separately from the single node to the microcontroller, help to prevent electromagnetic noise coupling among them. The return paths from the microcontroller use the generous grounding structure available with a two-sided circuit board. This provides a short run for the signal returns, in order to further help reduce radiated emissions. However, such designs known to the inventor still have T-filters incorporated on the circuit board across each regulated voltage input/return pair in order to assure reasonable radiated emission performance.
Using a single-sided circuit board and minimizing the number of filters required would greatly reduce the cost of a stepper motor instrument cluster. However, a challenge in the use of a single-sided board for this purpose is that a single-point "star" configuration as discussed above cannot readily be used; a single-sided board simply does not provide enough circuit-trace-routing flexibility to allow a star configuration. Further, the ability to provide generous grounds on two sides of the board is inherently eliminated when a single-sided board is used.
A design for a microcontroller-based electronic device which can use a single-sided circuit board and still provide excellent radiated emissions performance with the inclusion of only a minimal number of "T" filters will provide significant advantages over the prior art. Such a design will provide very substantial cost advantage over alternative designs.